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Light-value system
The Light-value system is a device, usually found on the lens barrels of leaf-shutter cameras of the late 1950s, that couples the speed and aperture rings. It allows choice among the various aperture–speed combinations that result in the same exposure by a single twist of both rings together, with no need for separate adjustments of aperture and speed. It was meant to be used together with exposure meters giving a reading in the Light-Value (LV) scale, that you could directly transfer to a LV dial on the lens, thus setting the correct exposure. The inconvenience is that you must unlock the device to change only one of the two parameters, or simply to change the exposure. This system was made obsolete by the direct coupling of the camera's internal exposure meter to the aperture and speed controls. The ability to switch the aperture and speed together was only retained on some cameras' lens barrels having the two rings side by side and with equidistant stops, such that the two rings can easily be turned together if and when desired. =The concept= Before dealing with the practical use in photography, it might be useful to look at the terms involved measuring light. The light value, denoted LV, refers to how bright a subject is in absolute terms, meaning how much light is coming from a subject, called its luminance. Pointing directly at the subject, some exposure meters give readings plainly in LV numbers. When the film-speed is properly set, exposure-time and lens-aperture combinations may be shown as well. *''LV = brightness reading off the subject'' Definition The LV is a logarithmic unit used for the measurement of subject brightness. In photography, LV expresses the luminance of an object or scene, enabling the brightness to be dealt with in absolute terms regardless of camera settings and emulsion or imaging-sensor light sensitivity. The normal range of LV is from 1 to 18, but it is a limitless scale, and may be expanded as required, like …-3, -2, -1, 0, 1, 2, 3, etc. Exposure Value In order to use the above information to take a picture, the light sensitivity of the film or imaging-sensor in the camera must be considered. An exposure meter with the relevant ISO/ASA-value set, can show the required exposure value, denoted EV, if so calibrated. The relationship between LV and EV is defined at ISO100, at which point the EV-value equals the LV-value. Increasing the ISO-value requires less amount of light for the same exposure and vice versa. *''EV = LV × ISO / 100'' On the camera The EV-value expresses the required exposure, regardless of aperture and shutter speed as long as the combination gives the same result. Changing the EV-value changes the exposure in the same amount. Within the limits of the available camera settings, several combinations might give the same exposure, the widest selection of combinations being at moderate EV-values, like EV9. Implementing this system on a mechanical camera meant fashioning a device for interlocking the shutter dial and aperture ring in such a way that, regardless of any change made to either, the exposure stays the same. Setting the exposure is accomplished by temporarily releasing the interlock, and select a different EV-value (or an associated shutter-aperture combination). On digital cameras, the whole process is programmed and performed automatically, if so desired. *''EV = camera settings giving the same exposure'' Examples EV0, as defined, requires exposure setting of f/1 at 1 sec. at ISO/ASA 100 on the camera. This equals f1.4 at 2sec., f/2 at 4sec., or f2.8 at 8sec. etc. EV1 requires half the amount of exposure, EV2 a quarter and EV3 an eighth and so on. Each full stop on the aperture scale (1.4 - 2 - 2.8 - 4 etc) represents a doubling or halving the exposure, as do doubling or halving the shutter speed (1/30 - 1/60 - 1/125 etc). *''EV1 = f/1.4 and 1 second at ISO/ASA 100''